I keep my water level up to about a quarter of an inch away from the rim of the tank. Your fish will be fine it you raise your water level to within an inch or so to the top rim. If you are worried about oxygen content of the water you can add and air pump, but it really shouldn't be a problem. This way they will have more room to swim and the current caused by the water won't push them around so much. You had better do it now, before you get a waterline 4 inches down your tank. It will be really obvious and ugly when you decide to put the water level up some more.
As for the plant, you can plant it outside or put it inside in a pot as a houseplant.
What kind of hood do you have? Does it allow for 2 bulbs at one time? A 15 watt isn't much light over a 12 gallon tank. You could always try some low light plants like anubias, java fern, java moss, and cryptocorynes.
nope, my present light doesnt allow for 2 tubes, im thinking of modifying my present system to accomodate 2 tubes. how much light would be 'enough'? and what colour would be good?
Two 15 watt tubes would be enough for many plants. The problem is that I don't think you can get the proper color temp in those tubes. It might be better to build DIY a top that can hold 2 screw-in compacts. If you look around they now have them in 65K or close to it. Depending on taste (different temperature bulbs make your tank look different) anything from 5500 - 6700 (many people use higher) will work fine.